Your comment, Knygatin, will receive, I hope, patient and reflective responses. I'll try to advance the discussion a little. Here are some thoughts.
1.The idea you imply, that a writer will write better supernatural stories if he (=the male or female author) believes in the supernatural, seems, at first sight at least, likely to be true. I don't know if it could really be proven. First, we would need to establish what are the (qualitatively) best stories in the genre. There'd probably be agreement about some favorites -- I suppose almost everybody who reads a lot of supernatural horror and has read, say, "The White People," likes it (but some might find it lacking in the graphic visceral quality they like...) -- BUT there might be a lot of disagreement about which stories are good apart from a small canon of classics. Then we would have to try to find out what the authors of those stories believed. But a second problem is that we don't know what the authors really believed, at least in some cases. What I mean is that there are probably a few authors who are on record about what they believed -and- are writing accurately, with sufficient self-knowledge, about the matter. I expect that when Blackwood went on record as saying he believed in, and practiced, certain disciplines that (he believed) enabled him to achieve a greater state of awareness, which revealed the "supernatural," he was sincere. But I don't -really- know that, and perhaps Blackwood himself, "deep down," believed something different. I know no reason to doubt that he did know what he really believed and that he was telling the truth about the matter, but I don't know. But to hold that the writer of the best supernatural stories must believe in the supernatural would require me to know that sort of thing, wouldn't it? Perhaps I'm making too much of these things, but I do feel like there are a couple of ambiguous matters here.
2.Nevertheless, I think you have a good point -- up to a point. Machen seems to have grown up as an orthodox Christian who soon rebelled against "puritanism" (to use the term loosely). He became convinced of the reality of the paranormal at least. He was some kind of Christian most of his life. (See the discussion of the academic paper "Man Is Made a Mystery" on machen's thought: [
www.sffchronicles.com]) Haggard seems to have believed in an eclectic set of beliefs including elements of Christianity and also reincarnation, etc. Blackwood did believe in the supernatural or the preternatural. M. R. James was, so far as I know, an orthodox Christian, and he also believed (which not all orthodox Christians do) in the possibility of "ghosts." I don't know a lot about de la Mare, but I think he did regard reality as something elusive, not just material, and not susceptible of being pinned down by empiricism. Hodgson (if my impression is correct) held to some kind of non-Christian spirituality. Wells may have believed in some kind of godlike potential for mankind if evolution wasn't defeated by us destroying ourselves, but I don't think he believed in the supernatural. Doyle, as everyone knows, became a public defender of occultism, the reality of fairies, etc. I don't know what Morris's beliefs were. As a socialist, he presumably thought largely in terms of the material conditions of life affecting or even determining society and the beliefs of individuals. George MacDonald was an ardent, but unorthodox, Christian. I have the impression that Dunsany didn't believe in the supernatural. So I think, Knygatin, that you could support your point about the Golden Age authors with quite a bit of biographical information.
3.It seems to me that the contemporary authors, whom I mostly don't read, don't have the sense of a transcendent or spiritual dimension. This helps to propel them towards more physical horror. Back in the 1980s I read several of Stephen King's novels, and was struck by how often the horror involved severe injury, gruesome death, etc. I suppose that this is what we find in the books of many other authors and in movies. The question may then be asked, as to whether being mutilated and bleeding out thanks to a bomb planted by a Mohammedan terrorist on a subway platform or being clawed to death by a zombie are all that different.
4.This brings me to Clark Ashton Smith. I'm curious about his beliefs, if it is possible to say much about them, but don't know a lot about his life. My sense is that he was probably close to the materialist Lovecraft, though more attracted to "decadence" than HPL. But I would guess that CAS didn't believe in the supernatural. His stories are replete with gruesome physical outrages, and very unlike the stories of Blackwood or Machen, say.